I'm not sure if anyone has tried the preview of Windows 8 but Dolphin works so far in the preview.
I know it's still awhile before they release it but it looks good and the speed is as fast as it is on Windows 7.

A lot of things could interfere not with Dolphin itself but with general application performance imo... I mean, can't one Windows be more optimized than the other when running the same program?
Not to talk about system requirements..